Processes have been developed for the preservation, drying and coloring of wood. Such processes are described in Swedish Pat. No. 301,870, U.S. Pat. No. 3,560,251, and Canadian Pat. No. 842,194. Such processes include an initial pressure treatment of the wood with an aqueous preservative-containing solution. The treated wood is thereafter dried in warm oil under a vacuum. The oil provides the wood with a water repellent surface. The wood can also be colored by pigmenting the oil.
Similar processes have also been developed where the treatment with the preservative is carried out with the preservatives being dissolved in organic solvents. The solvent is thereafter evaporated from the wood in warm oil under a vacuum. It was initially believed that the solvent and the wood-drying oil had to be insoluble in each other to achieve full evaporation. Experiments have shown, however, that this point of view has not been correct under certain conditions as described in Swedish Pat. No. 401,994, U.S. Pat. No. 3,995,077, and Canadian Pat. No. 1,051,292.
However, such prior art processes have certain disadvantages.
For example, in the first described prior art process an ammoniacal solution containing copper compounds has been used as a preserving agent. The copper compounds provide effective protection. The solution does not attack iron and the treatment apparatus remains relatively clean. The water-soluble copper compounds, however, impart a green color to the wood. This is a definite disadvantage as it makes it impossible to apply lighter colors while also limiting the possibility of employing other desired colors. The copper compounds further exhibit a destructive effect upon the oils, and especially upon the drying oils (e.g., linseed oil). Drying oils are desirably present in the wood-drying oil mixture to obtain a more permanent coloring of the wood. Even ammonia causes a certain coloring of the wood while also influencing the linseed and the other drying oils. After being used for some time the oil mixture is no longer fit for use and must be replaced. Typically such oil mixtures are replaced after 150 operations or so.
In the second described prior art process wherein an organic solvent solution is used in the preservation treatment, the coloring result obtained is not of the same high quality as when the treatment is carried out with water soluble preservatives. For this reason the first process has generally been preferred commercially.
However, in the process of the present invention, preservatives free of oil-attacking copper and ammonia can be used since the treatment solution is in the form of an emulsion. The preservatives of the present invention are less prone to attack the oils. The use of an emulsion with a high content of water also provide satisfactory coloring of the wood. Another advantage is that no evaporating solvents are needed to dissolve the preservative. In addition, the emulsifier is relatively inexpensive.